Add stubs for keywords not documented in https://doc.rust-lang.org/std/#keywords yet

[petrochenkov]

The keyword list that we can observe now leaves a pretty bad impression.
It doesn't say that it's a work in progress, it just provides an incorrect list.

When you see this list on the documentation landing page questions arise:
If some keyword is not in the list, is it actually a keyword?
Is the any reason and intent behind a specific keyword missing?

And the answer (perhaps not expected from a respectable project) is that there's no intent.
I seems like some initial keywords were added as a proof of concept, but then everyone forgot about this doc section.

Having "please document me" stubs may motivate people encountering them to complete the work.

[petrochenkov]

The documentation source can be found in src/libstd/keyword_docs.rs.
The keyword list can be found in libsyntax_pos/symbol.rs.

[ollie27]

This is part of #34601.

[timvisee]

Some effort has been made in #60761